A couple of weeks ago I was walking in Worcestershire and discovered waymarked posts for the Millennium Way. I noted the website http://www.walking.41club.org and discovered the path was part of a 100 mile walk from Pershore to Middleton Cheney via Meriden and passing through Worcestershire, Warwickshire and Northamptonshire which was completely missing from OSM.
Since then I have walked the western end of the route from Pershore to Stoulton and Broughton Hackett. In the next month or so I intend to continue as far as Inkberrow or Coughton. The route continues through Henley in Arden, HockleyHeath, Meriden, Kenilworth, Leamington Spa, Cubbington, Long Itchington, Priors Marston, Chipping Warden to Middleton Cheney so if it runs near where you are working please look out for it and map it.
The route was completely new to OSM so I created a relation for the way http://www.openstreetmap.org/?relation=1518176 and have added its details to the WikiProject United Kingdom Long Distance Paths http://wiki.openstreetmap.org/wiki/WikiProject_United_Kingdom_Long_Distance_Paths
The official walk website has full descriptions and sketch maps from west to east and east to west so you don't need to buy a book to walk just small sections. The site also includes short walks that cover most of the route and return by a other paths.